The objective of this study is to determine which features of ECE, from infancy through preschool, together with family experiences, predict the development of prosocial behavior. Data from the NICHD Study of Early Child Care and Youth Development will be used to address this objective. This national sample of more than 1200 families was followed from birth through adolescence, using an extensive array of well-validated, reliable questionnaire, interview, and observational measures of child, family, and ECE environments. Using SEM, the study will model how structural and process quality in ECE in infancy and toddlerhood (6 - 24 months of age) as well as over the preschool years (through 54 months of age) predict children's prosocial behavior in 1st grade, and how the associations vary as a function of amount and type of ECE that children experienced. The study will control for child characteristics that are likely to influence socialization processes in ECE, and for family characteristics that are known to influence children's ECE experiences. Family SES as a potential moderator of ECE effects will be examined. Second, the study will model how static and dynamic family factors in infancy and toddlerhood (6 - 24 months) as well as over the preschool years (through 54 months) predict prosocial behavior in 1st grade, and how these associations are affected by ECE experiences previously identified as predictive, with child characteristics again controlled. Finally, the patterns of growth (trajectories) in prosocial behavior over three time periods: prior to school entry; K - grade 6; grade 7 - 15 years will be examined. By providing meaningful, well-grounded, robust and generalizable answers to these important questions this study will inform future practice-relevant research that seeks to enhance positive developmental outcomes for children in ECE and inform policy-relevant issues concerning the role of universal provision of ECE as a means to advance social success and community welfare.